canonical "NaN" (#1744)

Signed-off-by: John Belmonte <john@neggie.net>

content/docs/instrumenting/exposition_formats.md

@@ -82,7 +82,7 @@ In the sample syntax:
 
 *  `metric_name` and `label_name` carry the usual Prometheus expression language restrictions.
 * `label_value` can be any sequence of UTF-8 characters, but the backslash (`\`), double-quote (`"`), and line feed (`\n`) characters have to be escaped as `\\`, `\"`, and `\n`, respectively.
-* `value` is a float represented as required by Go's [`ParseFloat()`](https://golang.org/pkg/strconv/#ParseFloat) function. In addition to standard numerical values, `Nan`, `+Inf`, and `-Inf` are valid values representing not a number, positive infinity, and negative infinity, respectively.
+* `value` is a float represented as required by Go's [`ParseFloat()`](https://golang.org/pkg/strconv/#ParseFloat) function. In addition to standard numerical values, `NaN`, `+Inf`, and `-Inf` are valid values representing not a number, positive infinity, and negative infinity, respectively.
 * The `timestamp` is an `int64` (milliseconds since epoch, i.e. 1970-01-01 00:00:00 UTC, excluding leap seconds), represented as required by Go's [`ParseInt()`](https://golang.org/pkg/strconv/#ParseInt) function.
 
 #### Grouping and sorting