Add Replace A Character as a vim til.

vim/replace-a-character.md

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+# Replace A Character
+
+Throughout the day I'll often find myself deleting a single character and
+putting a different one in its place. I usually navigate over the target
+character and hit `s` which removes the character under the cursor and puts
+me in insert mode. From there I type the new character and hit escape to
+return to normal node. This isn't the best way to perform such an edit
+though. Vim has a command specifically for replacing a character. The `r`
+command. It does essentially the same thing as my current approach but
+instead of putting me in insert mode, it simply replaces the character and
+leaves me in normal node.
+
+See `:h r` for more details.