Including `:` in the command names in docs

This will ensure the correct tag is used when we process readme to create the
vimdoc help file. See #330.

README.md

@@ -452,7 +452,7 @@ yours truly.
 Commands
 --------
 
-### The `YcmForceCompileAndDiagnostics` command
+### The `:YcmForceCompileAndDiagnostics` command
 
 Calling this command will force YCM to immediately recompile your file
 and display any new diagnostics it encounters. Do note that recompilation with
@@ -462,7 +462,7 @@ blocked.
 You may want to map this command to a key; try putting `nnoremap <F5>
 :YcmForceCompileAndDiagnostics<CR>` in your vimrc.
 
-### The `YcmDiags` command
+### The `:YcmDiags` command
 
 Calling this command will fill Vim's `locationlist` with errors or warnings if
 any were detected in your file and then open it.
@@ -471,18 +471,18 @@ A better option would be to use Syntastic which will keep your `locationlist`
 up to date automatically and will also show error/warning notifications in Vim's
 gutter.
 
-### The `YcmShowDetailedDiagnostic` command
+### The `:YcmShowDetailedDiagnostic` command
 
 This command shows the full diagnostic text when the user's cursor is on the
 line with the diagnostic.
 
-### The `YcmDebugInfo` command
+### The `:YcmDebugInfo` command
 
 This will print out various debug information for the current file. Useful to
 see what compile commands will be used for the file if you're using the semantic
 completion engine.
 
-### The `YcmCompleter` command
+### The `:YcmCompleter` command
 
 This command can be used to invoke completer-specific commands.  If the first
 argument is of the form `ft=...` the completer for that file type will be used