docs: use standard-compliant HTML entities for the four arrows

HTML 4 knows only the four-letter abbreviations for the arrows.
Use those, so that also browsers with very limited knowledge
(like Dillo) will show the arrows correctly.

doc/cheatsheet.html

@@ -79,14 +79,14 @@
 <table><tbody>
 <tr><td>Ctrl+B &nbsp;&nbsp;</td><td>One character backward</td></tr>
 <tr><td>Ctrl+F</td><td>One character forward</td></tr>
-<tr><td>Ctrl+<b>&leftarrow;</b></td><td>One word backward</td></tr>
-<tr><td>Ctrl+<b>&rightarrow;</b></td><td>One word forward</td></tr>
+<tr><td>Ctrl+<b>&larr;</b></td><td>One word backward</td></tr>
+<tr><td>Ctrl+<b>&rarr;</b></td><td>One word forward</td></tr>
 <tr><td>Ctrl+A</td><td>To start of line</td></tr>
 <tr><td>Ctrl+E</td><td>To end of line</td></tr>
 <tr><td>Ctrl+P</td><td>One line up</td></tr>
 <tr><td>Ctrl+N</td><td>One line down</td></tr>
-<tr><td>Ctrl+<b>&uparrow;</b></td><td>To previous block</td></tr>
-<tr><td>Ctrl+<b>&downarrow;</b></td><td>To next block</td></tr>
+<tr><td>Ctrl+<b>&uarr;</b></td><td>To previous block</td></tr>
+<tr><td>Ctrl+<b>&darr;</b></td><td>To next block</td></tr>
 <tr><td>Ctrl+Y</td><td>One page up</td></tr>
 <tr><td>Ctrl+V</td><td>One page down</td></tr>
 <tr><td>Alt+\</td><td>To top of buffer</td></tr>
@@ -98,8 +98,8 @@
 <table><tbody>
 <tr><td>Alt+G &nbsp;&nbsp;&nbsp;</td><td>Go to specified line</td></tr>
 <tr><td>Alt+]</td><td>Go to complementary bracket</td></tr>
-<tr><td>Alt+<b>&uparrow;</b></td><td>Scroll viewport up</td></tr>
-<tr><td>Alt+<b>&downarrow;</b></td><td>Scroll viewport down</td></tr>
+<tr><td>Alt+<b>&uarr;</b></td><td>Scroll viewport up</td></tr>
+<tr><td>Alt+<b>&darr;</b></td><td>Scroll viewport down</td></tr>
 <tr><td>Alt+&lt;</td><td>Switch to preceding buffer</td></tr>
 <tr><td>Alt+&gt;</td><td>Switch to succeeding buffer</td></tr>
 </tbody></table>