DOC: Tweak superscript formatting.

Note: vertical-alignment is off when line-height for superscripts is not 0.

doc/source/themes/agogo/static/agogo.css_t

@@ -599,6 +599,11 @@ pre {
     border-right: none;
 }
 
+sup {
+    font-size: x-small;
+    line-height: 0;
+}
+
 tt {
     background-color: #ecf0f3;
     padding: 0 1px 0 1px;

doc/source/user_guide/data_types.txt

@@ -8,17 +8,17 @@ data types [1]_, *i.e.* "dtypes". To avoid distorting image intensities (see
 `Rescaling intensity values`_), we assume that images use the following dtype
 ranges:
 
-=========  ===============================
+=========  =================================
 Data type  Range
-=========  ===============================
+=========  =================================
 uint8      0 to 255
 uint16     0 to 65535
 uint32     0 to 2\ :sup:`32`
 float      0 to 1
 int8       -128 to 127
 int16      -32768 to 32767
-int32      -2\ :sup:`31` to 2\ :sup:`31`-1
-=========  ===============================
+int32      -2\ :sup:`31` to 2\ :sup:`31` - 1
+=========  =================================
 
 Note that float images are restricted to the range 0 to 1 even though the data
 type itself can exceed this range; all integer dtypes, on the other hand, have