Fix MQTT link text to be sentence cased (home-assistant#157191)

Author: jbouwh

homeassistant/components/mqtt/strings.json

@@ -274,7 +274,7 @@
             "options": "Options for allowed sensor state values. The sensor’s Device class must be set to Enumeration. The 'Options' setting cannot be used together with State class or Unit of measurement.",
             "reports_position": "Set this option if the valve reports the position or supports setting the position. Enabling this option will cause the position to be published instead of a payload defined by payload \"open\", payload \"close\" or payload \"stop\". When receiving messages, state topic will accept numeric payloads or one of the configured state messages.",
             "schema": "The schema to use. [Learn more.]({url}#comparison-of-light-mqtt-schemas)",
-            "state_class": "The [State class]({available_state_classes_url}) of the sensor. [Learn more.]({url}#state_class)",
+            "state_class": "The [state class]({available_state_classes_url}) of the sensor. [Learn more.]({url}#state_class)",
             "suggested_display_precision": "The number of decimals which should be used in the {platform} entity state after rounding. [Learn more.]({url}#suggested_display_precision)",
             "supported_features": "The features that the entity supports.",
             "temperature_unit": "This determines the native unit of measurement the MQTT device works with.",