Since MapMarkers labels can only be 1 character, change Label property to char instead of string

[MaximillianC]

The Label property on MapMarkers is currently a string, but the StaticMaps API only allows labels to be a single uppercase character or digit, per below. So it would prevent some issues to change that property to be a character type instead of a string.

label: (optional) specifies a single uppercase alphanumeric character from the set {A-Z, 0-9}. (The requirement for uppercase characters is new to this version of the API.) Note that default and mid sized markers are the only markers capable of displaying an alphanumeric-character parameter. tiny and small markers are not capable of displaying an alphanumeric-character.