minor: fix article in comment

Author: tusharpm

include/fg/chart.h

@@ -223,7 +223,7 @@ FGAPI fg_err fg_add_histogram_to_chart(fg_histogram* pHistogram, fg_chart pHandl
                                        const unsigned pNBins, const fg_dtype pType);
 
 /**
-   Create and add an Pie object to the current chart
+   Create and add a Pie object to the current chart
 
    \param[out] pPie is the handle of the pie object created
    \param[in] pHandle is chart handle
@@ -482,7 +482,7 @@ class Chart {
         FGAPI Histogram histogram(const unsigned pNBins, const dtype pDataType);
 
         /**
-           Create and add an Pie object to the current chart
+           Create and add a Pie object to the current chart
 
            \param[in] pNSectors is number of sectors the data is sorted out
            \param[in] pDataType takes one of the values of \ref dtype that indicates