Fix inconsistency of input field placeholder text capitalization

The board search input field of the "Select Other Board and Port" dialog uses placeholder text to explain the usage of
the field to the user.

All other placeholder text in the IDE's UI uses sentence case. This specific placeholder was the exception, using
unpleasant caps lock instead.

The inconsistency is resolved by changing the placeholder text to the standard sentence case.

arduino-ide-extension/src/browser/boards/boards-config.tsx

@@ -308,7 +308,7 @@ export class BoardsConfig extends React.Component<
             className="theia-input"
             placeholder={nls.localize(
               'arduino/board/searchBoard',
-              'SEARCH BOARD'
+              'Search board'
             )}
             onChange={this.updateBoards}
             ref={this.focusNodeSet}