man: rename file to *.plugin.zsh (#6016)

Also fixed minor typo

Closes #6108

Co-authored-by: Matt <sifrawr@gmail.com>

plugins/man/man.plugin.zsh

@@ -5,7 +5,7 @@
 # * Jerry Ling<jerryling315@gmail.com>
 #
 # ------------------------------------------------------------------------------
-# Usgae
+# Usage
 # -----
 #
 # man will be inserted before the command
@@ -24,4 +24,4 @@ bindkey "\e"man man-command-line
 # ------------------------------------------------------------------------------
 # Also, you might want to use man-preview included in 'osx' plugin
 # just substitute "man" in the function with "man-preview" after you included OS X in
-# the .zshrc
+# the .zshrc