Change "install" to "activate" to make omf theme doc clearer (#816)

* As omf theme <name> won't install a theme per se, changing "install" to "activate" seems clearer. So omf theme will list installed and available themes, activate a installed one, but won't install one.

* As omf theme <name> won't install a theme per se, changing "install" to 
"activate" seems clearer. So omf theme will list installed and available 
themes, activate a installed one, but won't install one.

docs/cli/omf.adoc

@@ -20,7 +20,7 @@ Provides options to list, download and remove packages, update the framework, cr
 <<remove.adoc#,**r**emove>>::        Remove a package.
 <<repositories.adoc#,**repo**sitories>>::  Manage package repositories.
 <<search.adoc#,**s**earch>>::        Search for a package or theme.
-<<theme.adoc#,**t**heme>>::         Install and list themes.
+<<theme.adoc#,**t**heme>>::         Activate and list available themes.
 <<update.adoc#,**u**pdate>>::        Update Oh My Fish.
 <<version.adoc#,**version**>>::       Display version and exit.
 

docs/cli/theme.adoc

-Install and list themes.
+Activate and list available themes.
 
 == USAGE
-  omf theme         List available themes to install
-  omf theme <name>  Install theme by name
+  omf theme         List available and installed themes
+  omf theme <name>  Activate theme by name
 
 == EXAMPLES
   omf theme

pkg/omf/completions/omf.fish

@@ -45,6 +45,6 @@ complete -c omf -f -n "__fish_seen_subcommand_from repo repositories; and __omf_
 complete -c omf -f -n "__fish_seen_subcommand_from repo repositories; and __omf_assert_args_count 1" -a list -d "List installed repositories"
 complete -c omf -f -n "__fish_seen_subcommand_from repo repositories; and __omf_assert_args_count 1" -a remove -d "Remove a package repository"
 complete -c omf -f -a search   -n "__fish_use_subcommand" -d "Search for a plugin or theme"
-complete -c omf -f -a theme    -n "__fish_use_subcommand" -d "Install and list themes"
+complete -c omf -f -a theme    -n "__fish_use_subcommand" -d "Activate and list available themes"
 complete -c omf -f -a update   -n "__fish_use_subcommand" -d "Update Oh My Fish"
 complete -c omf -f -a version  -n "__fish_use_subcommand" -d "Display version"

pkg/omf/spec/basic_spec.fish

@@ -18,7 +18,7 @@ function describe_basic_tests
     echo $output | grep -Eq "remove.+Remove a package"
     echo $output | grep -Eq "repositories.+Manage package repositories"
     echo $output | grep -Eq "search.+Search for a package or theme"
-    echo $output | grep -Eq "theme.+Install and list themes"
+    echo $output | grep -Eq "theme.+Activate and list available themes"
     echo $output | grep -Eq "update.+Update Oh My Fish"
     echo $output | grep -Eq "version.+Display version and exit"
     assert 0 = $status