spec: Fix a code tag

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/1266042

doc/go_spec.html

@@ -3297,7 +3297,7 @@ For instance, the value of a variable <code>x</code> of type <code>float32</code
 may be stored using additional precision beyond that of an IEEE-754 32-bit number,
 but float32(x) represents the result of rounding <code>x</code>'s value to
 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits
-of precision, <code>but float32(x + 0.1)</code> does not.
+of precision, but <code>float32(x + 0.1)</code> does not.
 </li>
 </ol>