Fix incorrect environment variable name in README

The README references the environment variable `STDLIB_LOG_DEPRECATION`
while the code uses `STDLIB_LOG_DEPRECATIONS` (note the trailing S).

Author: smoeding

README.markdown

@@ -358,7 +358,7 @@ deprecation(key, message)
 
 The Puppet settings '[disable_warnings](https://docs.puppet.com/puppet/latest/reference/configuration.html#disablewarnings)', '[max_deprecations](https://docs.puppet.com/puppet/latest/reference/configuration.html#maxdeprecations)', and '[strict](https://docs.puppet.com/puppet/latest/reference/configuration.html#strict)' affect this function. Set 'strict' to `error` to fail immediately with the deprecation message, `off` to output emit no messages at all, or `warning` (default) to log all warnings.
 
-Additionally you can set the environment variable `STDLIB_LOG_DEPRECATION` to decide whether or not to log deprecation warnings: if this environment variable is set to `true`, the functions log a warning, if it is set to `false`, no warnings are logged. If no value is set at all, Puppet 4 will emit warnings, while Puppet 3 will not. Using this setting is especially useful for automated tests to avoid flooding your logs before you are ready to migrate.
+Additionally you can set the environment variable `STDLIB_LOG_DEPRECATIONS` to decide whether or not to log deprecation warnings: if this environment variable is set to `true`, the functions log a warning, if it is set to `false`, no warnings are logged. If no value is set at all, Puppet 4 will emit warnings, while Puppet 3 will not. Using this setting is especially useful for automated tests to avoid flooding your logs before you are ready to migrate.
 
 *Type*: String, String.