fixfeat(mappings): add new undo history mapping

Author: mehalter

src/content/docs/mappings.mdx

@@ -213,6 +213,7 @@ AstroNvim generally relies on `<Leader>` driven mappings, which is default set t
 | Registers                         | `Leader + fr`        |
 | Smart (buffers,recent,files)      | `Leader + fs`        |
 | Colorschemes                      | `Leader + ft`        |
+| Undo History                      | `Leader + fu`        |
 | Live Grep                         | `Leader + fw`        |
 | Live Grep (include hidden files)  | `Leader + fW`        |
 | Git Branches                      | `Leader + gb`        |