docs: use more generally available arrows in the cheatsheet

Use bolded HTML entities for the four arrows, instead of the literal
triangular Unicode "arrows" which many fonts don't include.

This addresses https://savannah.gnu.org/bugs/?57505.

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+⯇</td><td>One word backward</td></tr>
+<tr><td>Ctrl+<b>&leftarrow;</b></td><td>One word backward</td></tr>
-<tr><td>Ctrl+⯈</td><td>One word forward</td></tr>
+<tr><td>Ctrl+<b>&rightarrow;</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+⯅</td><td>To previous block</td></tr>
+<tr><td>Ctrl+<b>&uparrow;</b></td><td>To previous block</td></tr>
-<tr><td>Ctrl+⯆</td><td>To next block</td></tr>
+<tr><td>Ctrl+<b>&downarrow;</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+⯅</td><td>Scroll viewport up</td></tr>
+<tr><td>Alt+<b>&uparrow;</b></td><td>Scroll viewport up</td></tr>
-<tr><td>Alt+⯆</td><td>Scroll viewport down</td></tr>
+<tr><td>Alt+<b>&downarrow;</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>