add "zsh-" prefix to history-substring-search

@zsh-users/zsh-syntax-highlighting and
@zsh-users/zsh-history-substring-search don't follow the same naming
convention.

This commit just rename history-substring-search.zsh and
history-substring-search.plugin.zsh to zsh-history-substring-search.zsh
and zsh-history-substring-search.plugin.zsh respectively.

README

@@ -1,7 +1,7 @@
 To activate this script, load it into an interactive ZSH session:
 
-  % source history-substring-search.zsh
+  % source zsh-history-substring-search.zsh
 
-See the "history-substring-search.zsh" file for more information:
+See the "zsh-history-substring-search.zsh" file for more information:
 
-  % sed -n '2,/^$/s/^#//p' history-substring-search.zsh | more
+  % sed -n '2,/^$/s/^#//p' zsh-history-substring-search.zsh | more

zsh-history-substring-search.plugin.zsh

@@ -1,6 +1,6 @@
-# This file integrates the history-substring-search script into oh-my-zsh.
+# This file integrates the zsh-history-substring-search script into oh-my-zsh.
 
-source "$ZSH/plugins/history-substring-search/history-substring-search.zsh"
+source "$ZSH/plugins/zsh-history-substring-search/zsh-history-substring-search.zsh"
 
 if test "$CASE_SENSITIVE" = true; then
   unset HISTORY_SUBSTRING_SEARCH_GLOBBING_FLAGS

zsh-history-substring-search.zsh

@@ -10,13 +10,13 @@
 #
 # 1. Load this script into your interactive ZSH session:
 #
-#       % source history-substring-search.zsh
+#       % source zsh-history-substring-search.zsh
 #
 #    If you want to use the zsh-syntax-highlighting[6] script along with this
 #    script, then make sure that you load it *before* you load this script:
 #
 #       % source zsh-syntax-highlighting.zsh
-#       % source history-substring-search.zsh
+#       % source zsh-history-substring-search.zsh
 #
 # 2. Type any part of any previous command and then:
 #