chore(gem): minor formatting changes

Make minor formatting changes to the `gem` cheatsheet.

gem

@@ -1,10 +1,10 @@
 ---
-tags: [ packaging ]
+tags: [ packaging, ruby ]
 ---
 # To search for a package:
 gem search <package>
 
-# To install packages:
+# To install a package:
 gem install <package>
 
 # To install a package in user space:
@@ -13,7 +13,7 @@ gem install --user-install <package>
 # To install specific version of a package:
 gem install <package> -v <version>
 
-# To uninstall packages:
+# To uninstall a package:
 gem uninstall <package>
 
 # To upgrade a package:
@@ -25,5 +25,5 @@ gem info <package>
 # To list local packages:
 gem list --local
 
-# To show help on command
+# To show help on command:
 gem help <command>