docs: fix keybindings table column mismatch in README (#31)

The separator row had five columns while the header has four, breaking
GitHub's table rendering with a stray empty column.

Author: programmersd21

README.md

@@ -132,7 +132,7 @@ flow --help
 ### Keybindings
 
 | Key | Action | Key | Action |
-|---|---|---|---|---|
+|---|---|---|---|
 | `q` / `^C` | Quit | `c` | Cycle display units |
 | `m` | Cycle display modes | `b` | Toggle bits/bytes |
 | `d` | Cycle display filter | `+` / `-` | Adjust refresh interval |