agnoster: fix icon for unstaged git status (#9164)

themes/agnoster.zsh-theme

@@ -131,7 +131,7 @@ prompt_git() {
     zstyle ':vcs_info:*' get-revision true
     zstyle ':vcs_info:*' check-for-changes true
     zstyle ':vcs_info:*' stagedstr '✚'
-    zstyle ':vcs_info:*' unstagedstr '●'
+    zstyle ':vcs_info:*' unstagedstr '±'
     zstyle ':vcs_info:*' formats ' %u%c'
     zstyle ':vcs_info:*' actionformats ' %u%c'
     vcs_info