doc: dix typo

Fixes type at line 27 of doc/number.md.

PR-URL: #435
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

Author: abhi11210646

doc/number.md

@@ -24,7 +24,7 @@ Creates a new instance of a `Napi::Number` object.
 Napi::Number(napi_env env, napi_value value);
 ```
 
- - `[in] env`: The `napi_env` environment in which to construct the `Napi::Nuber` object.
+ - `[in] env`: The `napi_env` environment in which to construct the `Napi::Number` object.
  - `[in] value`: The `napi_value` which is a handle for a JavaScript `Number`.
 
  Returns a non-empty `Napi::Number` object.