Fixed argument type in switchToIFrame() to be integer.

[AlexSkrypnyk]

Release 1.11.0 introduced strict types.

With that, the switchToIFrame(string? $id) method started to fail on consumer projects using w3c = true because the expected $id argument has a type string instead of expected int.

According to https://www.w3.org/TR/webdriver/#switch-to-frame, the $id is expected to be an integer.

This PR updates string? with int?.

Note that downstream project MinkSelenium2Driver have the same issue propagated after the recent release 1.7.0: https://github.com/minkphp/MinkSelenium2Driver/blob/master/src/Selenium2Driver.php#L431

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d8527fd) 98.72% compared to head (95f5a97) 98.72%.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #863   +/-   ##
=========================================
  Coverage     98.72%   98.72%           
  Complexity      374      374           
=========================================
  Files            24       24           
  Lines          1022     1022           
=========================================
  Hits           1009     1009           
  Misses           13       13           

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