Fix a typo in the Espresso example README.md (#71)

diff --git a/examples/ex2_espresso/README.md b/examples/ex2_espresso/README.md
index bf3cb34..6d9053f 100644
--- a/examples/ex2_espresso/README.md
+++ b/examples/ex2_espresso/README.md
@@ -97,7 +97,7 @@
 
     ```python
     def test_click_button(self):
-      self.dut1.snippet.clickMainButton()
+      self.dut1.snippet.pushMainButton()
     ```
 
 ## Running the example code