[spec] Fix typo (#1458)

Author: tpmccallum

document/core/exec/runtime.rst

@@ -18,7 +18,7 @@ Runtime Structure
 Values
 ~~~~~~
 
-WebAssembly computations manipulate *values* of either the four basic :ref:`number types <syntax-numtype>`, i.e., :ref:`integers <syntax-int>` and :ref:`floating-point data <syntax-float>` of 32 or 64 bit width each, of :ref:`vectors <syntax-vecnum>` of 128 bit width, or of :ref:`reference type <syntax-reftype>`.
+WebAssembly computations manipulate *values* of either the four basic :ref:`number types <syntax-numtype>`, i.e., :ref:`integers <syntax-int>` and :ref:`floating-point data <syntax-float>` of 32 or 64 bit width each, or :ref:`vectors <syntax-vecnum>` of 128 bit width, or of :ref:`reference type <syntax-reftype>`.
 
 In most places of the semantics, values of different types can occur.
 In order to avoid ambiguities, values are therefore represented with an abstract syntax that makes their type explicit.