fix typo in readme

Author: matthewgoslett

README.md

@@ -155,7 +155,7 @@ $histogram = $exporter->registerHistogram(
 );
 // the buckets must be in asc order
 // if buckets aren't specified, the default 0.005, 0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10.0 buckets will be used
-$histogram->obeserve(5.0);
+$histogram->observe(5.0);
 
 // create a histogram (with labels)
 $histogram = $exporter->registerHistogram(
@@ -166,7 +166,7 @@ $histogram = $exporter->registerHistogram(
 );
 // the buckets must be in asc order
 // if buckets aren't specified, the default 0.005, 0.01, 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10.0 buckets will be used
-$histogram->obeserve(5.0, ['GET']);
+$histogram->observe(5.0, ['GET']);
 
 // retrieve a histogram
 $counter = $exporter->getHistogram('response_time_seconds');